<Acrylic Resin Plate>
Acrylic resin plates are used for lenses, automobile parts, illumination parts, various electronic displays and the like due to their excellent optical properties. However, acrylic resin plates have a demerit of insufficient heat resistance in the case of effecting heat treatment working at high temperatures.
As a technology of improving the heat resistance of an acrylic resin plate, there is a method of introducing a cross-link structure by adding a poly-functional monomer in polymerization of methyl methacrylate. For example, there is suggested a method in which a poly-functional (meth)acrylate of an alkylene glycol is added to a composition composed of a methyl methacrylate homopolymer and methyl methacrylate, and the mixture is casting-polymerized, for the purpose of improving mainly heat resistance and impact resistance (see, e.g., Japanese Patent Application Publication (JP-B) No. 4-75241). In this method, however, sufficient heat resistance cannot be obtained usually. For obtaining sufficient heat resistance by this method, a poly-functional (meth)acrylate should be added in large amount, and the appearance of the resulting resin molded article tends to deteriorate in this procedure.
Furthermore, there is suggested a method in which at least one of cyclohexadiene and derivatives thereof and terpenoid-based compounds and derivatives thereof is added to methyl methacrylate and a poly-functional (meth)acrylate and the mixture is casting-polymerized, for the purpose of improving heat resistance and appearance (see, e.g., Japanese Patent Application Laid-Open (JP-A) No. 2002-265538). In this method, however, sufficient heat resistance cannot be obtained usually. Additionally, the resulted resin plate is a resin plate liable to absorb moisture.
Furthermore, for the purpose of improving heat resistance and appearance, there is suggested a method of casting-polymerizing a composition comprising a cross-linking agent and an alkyl methacrylate-based syrup obtained by compounding an alkyl methacrylate monomer and a (meth)acrylate-based cross-linking agent and partially polymerizing this (see, e.g., JP-A No. 63-30510). However, in this method, gelling tends to occur in compounding a cross-linking agent to prepare a syrup.
For the purpose of improving appearance, there is suggested a method of determining a ratio of a cross-linking agent to an alkyl methacrylate-based polymer within a certain range (see, e.g., JP-A No. 61-225207). However, there is no example here in which the amount of a poly-functional monomer is over 20 wt %, and sufficient heat resistance cannot be obtained usually by this method. Furthermore, there is a restriction in composition for obtaining a resin plate having high heat resistance and excellent in appearance, generating a disturbance in industrialization.
Furthermore, there is suggested a method of producing an acrylic resin plate in which a monomer mainly composed of methyl methacrylate and allyl (meth)acrylate are casting-polymerized using at least two radical polymerization initiators having 10 hours half life temperatures of higher than 75° C. and lower than 75° C., the difference thereof being 5° C. or more (see, e.g., JP-A No. 9-25305). However, in this method, there is a tendency that polymerizability of an allyl group is poor and sufficient heat resistance is not obtained.
Still further, there is suggested a method of producing an optical material which is radical-polymerized using as a main component a poly-functional (meth)acrylate having an olefinic group (see, e.g., JP-B No. 4-30410). However, in this method, there is a problem that a plate cracks unless peeling is conducted at high temperatures in a peeling process in production.
<Transparent Electrode Plate for Touch Panel and Touch Panel>
Display integrated type input units having a transparent touch panel placed on a display such as a liquid crystal and cathode ray tube are used as an operation image plane of portable information terminals and cash dispensers in banks and the like since when the display image plane is touched with an input pen or finger, the touch panel acts as an input unit and input operation can be conducted easily. Particularly, analog touch panels of resist membrane mode are used most widely since these can be applied to all operation image planes.
Analog touch panels of resist membrane mode have in general a structure comprising an upper transparent electrode plate and a lower transparent electrode plate wherein the upper and lower transparent electrode plate are a transparent electrode plate having a transparent base plate and a transparent electrically conductive membrane formed on this transparent base plate and the upper and lower transparent electrode plates are placed at an interval so that the transparent electrically conductive membranes face mutually.
When an upper transparent electrode plate of a touch panel having such a structure is pressed with an input pen or finger, the upper transparent electrode plate is deflected and transparent electrically conductive membranes of the upper and lower transparent electrode plate contact mutually at this pressed point. The coordinate of this contact point is detected by measurement of electric resistance and input information is read.
As a transparent electrode plate of such a touch panel, there are generally used those in which a resin plate is used as a transparent base plate in the upper transparent electrode plate and a glass plate or resin plate is used as a transparent base plate in the lower transparent electrode plate and a transparent electrically conductive membrane is formed on the surface of these transparent base plates by a vacuum membrane forming method such as a vacuum vapor deposition method, sputtering method, CVD (chemical vapor deposition) method, and ion plating method.
However, the lower transparent electrode plate having a glass plate used as a transparent base plate has problems that it tends to crack in fabricating or carrying a touch panel or pressing a panel with a pen or hand, decreasing in thickness is difficult, decreasing in weight is difficult, and the like.
On the other hand, when a resin plate is used as a transparent base plate, problems of breakage of a base plate occurring in the case of use of a glass plate as a transparent base plate, and of decreasing in thickness and decreasing in weight can be easily solved. Actually, upper and lower electrode plates having a resin plate used as a transparent base plate are also variously investigated (see, for example, JP-A Nos. 2000-276301, 2001-14951 and 2001-34418). However, transparent base plates using a resin plate made of polyethylene terephthalate and the like disclosed in these patent literatures have insufficient transparency. Additionally, there are problems that due to lack of heat resistance, a base plate tends to thermally deform in forming a transparent electrically conductive membrane on the transparent base plate, and because of low close adherence and insufficient durability of a transparent electrically conductive membrane, the surface of a transparent base plate is required to be further processed, and the like.
Furthermore, there is disclosed a methacrylic resin molding material obtained by polymerizing, as a monomer, methyl methacrylate and neopentyl glycol dimethacrylate which is a poly-functional (meth)acrylate (see, e.g., JP-B No. 5-6570). However, there is utterly no disclosure that this methacrylic resin molding material can be utilized as a transparent base plate of a transparent electrode plate for touch panel, and there is utterly no suggestion what composition is suitable for a transparent electrode plate for touch panel, here. Furthermore, the methacrylic resin molding material described in this patent literature has a polymerization ratio of as low as 4 to 62 wt %, therefore, when this molding material is made into a product, it is necessary to enhance polymerization ratio by processes such as compression molding and extrusion molding. Consequently strain occurs, namely, this material is not suitable for a touch panel.
Even further, there is disclosed a touch panel using a transparent base plate obtained by preparing a photo-hardable composition by adding mercaptan to a monomer mixture containing a specific bis(meth)acrylate in an amount of 90 wt % or more, making the composition into a sheet in a cell, and photo-hardening this sheet (see, e.g., JP-A No. 10-105335). However, this transparent base plate has a problem that it tends to crack in a process of peeling from a cell after polymerization and hardening.